first of all for how long have you been taking tamoxifen? if it is a new medication, it is known to cause nausea
. If you have been taking this medication for a long time then I don't think it is the culprit.
Other options include what is called "small intestinal bacterial overgrowth", which can lead to increase gas production by this bacterial overgrowth and give you these symptoms, particularly burping, bloating
and ultimately nausea and vomiting. Delayed gastric emptying is another alternative, which is commonly seen in diabetics, but can also be seen without any other medical problems. It is diagnosed by a radiologic study called "gastric emptying study". You should be evaluated by a gastroenterologist for further recommendations.